Grapefruit may interact with tadalafil and lead to unwanted side effects. Some common side effects include headaches, back pain, nasal congestion, or flushing. Take the medicine at the same time each day, with or without food. It can also interact with many medicines, particularly nitrates used in some types of heart disease and blood pressure medicines. Tadalafil may not be safe for people with certain medical conditions, including people who have had a stroke , have coronary heart disease or retinitis pigmentosa an eye disease. Potential Side Effects and Interactions While Cialis is generally safe, like all medications, it can have side effects.The normal dosage of Cialis for occasional use in treating ED is 10 mg taken before sexual activity. If you have side effects with the mg dose, your doctor may prescribe a 5-mg dose. If the Roman sells the following versions of Cialis: Generic: available in tablets of 5, 10, and 20 milligrams (mg), starting from $11 Daily Generic: available in doses of and 5 mg at $8 Branded. What is Cialis? Cialis is an ED treatment that works in 30 minutes and lasts for up to 36 hours (about 1 and a half days). It contains the active ingredient tadalafil and it’s effective for 80% of men. How to take Cialis. Take one tablet at least 30 minutes before sex. The effects of Cialis can last for up to 36 hours.